Comparative physiology and transcriptome response patterns in cold-tolerant and cold-sensitive varieties of Zanthoxylum bungeanum Maxim

Zanthoxylum bungeanum is an economically important tree species of edible, medicinal, and ornamental value. The trees grow in varied climates from tropical to subtropical regions owing to their high resistance to adverse environment. The unstable environmental temperature is one of the important factors affecting the yield and quality of Z. bungeanum. To investigate complex responses to cold stress in Z. bungeanum, we compared cold-tolerant "Fuguhuajiao" (FG) and cold-sensitive "Fengxiandahongpao" (FX) varieties. Physiological measurements and transcriptome sequencing were performed using plant leaves collected at six different timepoints (0, 1, 3, 6, 12, and 24 h) of 4 °C treatment. Chemometrics analysis showed that the physiological cold response in FX at late stage was similar to that in FG at early stage. Based on orthogonal partial least squares discriminant analysis (OPLS-DA), the variable important in projection (VIP) values of peroxidase (POD) and soluble protein were 1.11 and 1.41, which were identified as important physiological indices in the two varieties. Under low temperature stress, there were 3513 differentially expressed genes in FG and 25,157 in FX. Weighted gene co-expression network analysis (WGCNA) revealed that many cold-response genes and pathways were enriched in the MEfirebrick4 module which had a high correlation with POD. Hub genes identified in this module indicated that Z. bungeanum can respond and adapt to cold stress by altering signal transduction, plant hormones, transcription factors, protein modification, functional proteins, metabolism, cell structure, light response, and the circadian clock. Endocytosis-related gene ERD7, abscisic acid pathway-related gene PP2C, late embryogenesis abundant gene LEA D-29, and two plasma membrane/lipid raft-associated remorin genes ZB01477 and pp34 are located at the core of the network, and all play pivotal roles in cold responses. These results of this study highlight cold response mechanisms at the physiological and molecular levels in Z. bungeanum and will contribute to a better understanding of low temperature resistance in related plants.